Engines, such as internal combustion engines, commonly utilize intake and exhaust valves associated with engine cylinders. In a normal running mode, intake valves may be opened to admit a fuel and air mixture into the combustion chamber of a cylinder, and exhaust valves may be opened to permit combustion byproducts to be exhausted from the cylinder.
It is also known to operate engines in a compression release braking mode, in which one or more engine cylinders are temporarily used to consume energy by compressing air. The energy required to compress air in the one or more engine cylinders produces a braking effect. This is accomplished by controllably opening a cylinder valve, for example, an exhaust valve, at a time when the engine has consumed energy compressing air in the cylinder, thus preventing recovery of the expended energy.
One common method for controllably opening an engine valve during a braking event is to use a rotating cam, for example, an injector cam, associated with the engine to provide the mechanical energy necessary to open the valve. The cam may be linked to the valve by a hydro/mechanical linkage including master and slave pistons and cylinders, such that rotation of the cam is translated into valve motion. One problem with this method is that the profile of the engine cam may not provide the optimum amount of valve displacement or lift. This is a particular problem in the braking mode of engine operation, because the engine cylinder valve is typically opened during the time that the cylinder piston is at or near top dead center. Consequently, if the valve extends too far into the cylinder it can make contact with the piston causing damage to the engine.
One approach to controlling the amount of valve displacement when using a fixed cam profile to indirectly actuate an engine valve is to employ some form of lost-motion linkage between the cam actuator and the engine valve. Such a linkage is designed to absorb motion caused by the fixed cam profile that exceeds the amount necessary to provide a desired valve displacement.
Many of the lost-motion devices employed in the past for this purpose incorporate some form of hydraulic arrangement, such that the force transmitted from the cam actuator to the engine valve is dissipated or absorbed hydraulically during at least a portion of the cam profile excursion. Such devices are sometimes referred to as hydraulic clippers, because they clip or limit excessive valve travel. For example, U.S. Pat. No. 6,415,752 illustrates a lost-motion system that includes a hydraulic accumulator that controllably absorbs hydraulic fluid in excess of that required to cause a desired engine valve displacement.
Known lost-motion devices used for the above described purpose tend to be complex and expensive, and often employ special and precise hydraulic fluid porting. This can lead to system failures and unreliable operation. The present invention is directed to overcoming one or more of the problems as set forth above.